1. Field of the Invention
The present invention relates to an optical encoder used for position measurement in a machine tool and the like.
2. Description of the Related Art
FIG. 1 is a perspective view of a typical optical encoder. There are disposed a light emitting unit TL composed of light sources LD for irradiating lights to a main scale MS which moves in the longitudinal direction, collimator lenses CL and a mirror MR, and a light beams receiving unit RL which is disposed opposite to the light emitting unit TL across the main scale MS in order to read the displacement of the main scale MS by an optical means. The main scale MS has gratings formed by repeated transmitting portions and non-transmitting portions and having a length corresponding to the longitudinal measurement length. Light beams emitted from the light sources LD such as laser diodes are respectively collimated by the respective collimator lenses CL, reflected by the mirror MR, and enter the main scale MS. It is possible to make the optical encoder compact by refracting the optical paths by means of the mirror MR as shown in FIG. 1.
In general, it is preferable that the area of a light beam which irradiates a main scale MS may be large since the larger the area is, the more the errors of gratings formed on the main scale MS can be properly averaged and decrease. Furthermore, in an absolute encoder, since a plurality of grating tracks are formed on the main scale MS and it is necessary to simultaneously irradiate the grating tracks, a light beam having a large irradiation area is necessary.
If the above-mentioned light beam having a large irradiation area is necessary in a conventional optical encoder, a plurality of sets of light sources LD and collimator lenses CL are arranged to irradiate the main scale MS as shown in FIG. 1. However, the grating tracks of the main scale MS is in the shape of a strip and a section of an effective light beam which is photoelectrically converted by the light receiving unit RL is rectangular, while the respective collimator lenses CL are circular. Therefore, the collimator lenses CL are required to be large in order to form an effective light beam having a rectangular section, and as a result, the light emitting unit TL is also large. For example, when a light beam having a rectangular section of 10 mm square is formed, collimator lenses CL having an effective aperture of 14 mm are necessary.
It is also required that a plurality of light beams emitted from the light emitting unit TL each be collimated and the optical axes of the light beams be parallel to one another. In order to meet these requirements, the performance of each of the collimator lenses CL should be excellent and the positions of the light sources LD and the collimator lenses CL should be accurately adjusted.